Various passive occupant restraint systems have been proposed to protect vehicle occupants, in case of a vehicle crash, from secondary impacts resulting from the collision of the occupants with the interior surfaces of the passenger compartment. Such passive occupant restraint systems include various forms of inflatable air bags and seat belt retractor systems which are capable of removing a slack in the seat belt. These passive occupant restraint systems require a sensor for detecting the occurrence of a crash.
Generally, acceleration sensors or deceleration sensors are used for such a purpose, and these sensors are mounted in the parts of the vehicles which are suitable for detecting an acceleration level indicative of a crash substantially without any time delay or which are relatively free from undesirable accidental activation. Accordingly, they may be arranged in the front parts of the vehicles, on the floor tunnels adjacent to the vehicle occupants and other appropriate locations. Therefore, they are often placed in poorly accessible locations. Further, considering the function of such sensors, it is difficult to routinely test the capability of the sensors to operate satisfactorily.
U.S. Pat. No. 4,536,629 discloses a gas damped acceleration switch in which a mass is supported by a spiral spring in such a manner that a contact set is closed only after the mass has traveled a certain distance against the spring by an inertia force acting on it. Further, damping means is provided to the mass so that the acceleration switch be insensitive to shocks of short durations and small magnitudes, and be prevented from actuating the passive occupant restraint system in conditions other than crash situations.
In such an acceleration switch, it is extremely important to assure the spring member for supporting the sensor mass to be in working order at all time. For instance, if there is any fracture or any other damage in the spring member, the threshold level of acceleration becomes inappropriate, and the acceleration switch becomes totally incapable of detecting such acceleration with any accuracy. This is significant all the more because it cannot be discovered unless a thorough testing of the acceleration switch is performed.